Working and functions of alternator in cars and starting current of cars

Alternator in Cars

Alternators are used in cars to generate current for running of cars. We quite often get confused that why alternator is required when there is battery available in cars.

Battery in cars is only used for starting the cars, but for lights operation and functioning of other parts current is required now from where that power will come???

For fulfilling the above purpose alternator is required, alternator has following functions in cars:-

(i)            Running of lights, heater, Air conditioners and operation of other electrical accessories

(ii)          Recharging of battery as it get discharged during starting.

Now how this alternator works in cars and who provides mechanical power to alternator so that power output can be generated at alternator output??

When you switched off your car engine than radio of car will work on car battery. For engines running following are required:-

(i)                    Air

(ii)                  Fuel

(iii)                 Spark

Last part is supplied by alternator because spark is generated through electricity,  although battery is available for supplying the spark but not for keep running the car. Battery electricity is sufficient for keep running the vehicle for few KMs  but vehicle required  much more than that , so to serve the purpose alternator will comes into picture.  Alternator in vehicle has output of 13.5 - 14.8 volts.

In past generators were used in cars, these generators are very inefficient in comparison to alternators, also at that time charging of battery and keeping accessories lighting up was different from present scenario. There are following components of alternators:-

(i)            Stator

(ii)          Rotor

(iii)         Voltage regulator you can also say automatic voltage regulator

(iv)         Direct current circuitry consisting of diodes

Now with rotation of rotor electricity get generated and output is used for charging batteries and keeping the auxiliaries ON.

DC circuitry is used to convert the alternating current generated by alternator to DC. Voltage regulator is used to keeping the voltage generated by alternator within limits.  Feedback is received by voltage regulator and accordingly output gets controlled.  Now day’s voltage regulators are integral part of alternators. But in past voltage regulators were big boxes and they were kept in hood and wired into the system. Voltage regulator functions are as below:-

(a)  Cut off the power to battery when battery get fully charged i.e. when battery voltage reached certain level around 14.0-14.5  volts

(b)  To keep voltage level within limits as per requirement of auxiliaries.

How to get noticed that Alternator is faulty??

It can be easily detected as you will observe reduced illumination from head lights , sometimes reduced illumination may not get detected while driving as during that time battery will provide necessary electricity . But as battery power get used for illumination of headlights now when you tried again to restart the vehicle then vehicle doesn’t start as battery get drained up.

Starting current of a car:-

When you tried to start a car your battery should be strong enough to provide crank to engines so that car get started easily. You may often see that very heavy leads are connected at battery terminals as starting current is very heavy for starting the car. By Ohms law we know that Voltage = Current X Resistance and now Voltage of battery is 12 V and starter motor resistance is approximately 0.12 Ohm so

Current = 12/ (0.12 + Internal resistance of battery)

Now lower is the resistance of battery higher will be the starting current. Now it is general practice that more expensive is the battery lower will be the resistance.

So higher will be the current and faster will be the starting of vehicle.

From above equation we will also find that if we neglect battery resistance then starting current will be 100A. So staring current will be also less than 100A always. If your battery is fully charged then it will be good enough to overcome the problem of poor internal resistance of battery. Battery is considered to be fully charged at 12.6 volt levels and low at 12.6 Volts and completely discharged at 11.9 Volts. Good quality batteries will able to start vehicle even when it is completely discharged so much but a poor quality battery may not be able to start vehicle even at 80% charged position.

In general when your car is new then you can buy any battery and don’t worry about anything but when your get older always buy a high quality battery, as resistance of older wires may get increased which may leads to lower starting current. So we have also seen that during starting current is very high approx. 100 A so it is always advisable to use strong leads capable of taking that 100A load.